1. Field of the Invention
The present invention relates to the transport and storage of semiconductor wafers, and in particular to an integrated system capable of transferring and stocking wafer-carrying pods between various processing tools within a bay of a semiconductor wafer fab, which system operates with a minimum amount of hardware and software control and in a space efficient manner.
2. Description of Related Art
A SMIF system proposed by the Hewlett-Packard Company is disclosed in U.S. Pat. Nos. 4,532,970 and 4,534,389. The purpose of a SMIF system is to reduce particle fluxes onto semiconductor wafers during storage and transport of the wafers through the semiconductor fabrication process. This purpose is accomplished, in part, by mechanically ensuring that during storage and transport, the gaseous media (such as air or nitrogen) surrounding the wafers is essentially stationary relative to the wafers, and by ensuring that particles from the ambient environment do not enter the immediate wafer environment.
The SMIF system provides a clean environment for articles by using a small volume of particle-free gas which is controlled with respect to motion, gas flow direction and external contaminants. Further details of one proposed system are described in the paper entitled "SMIF: A TECHNOLOGY FOR WAFER CASSETTE TRANSFER IN VLSI MANUFACTURING," by Mihir Parikh and Uhich Kaempf, Solid State Technology, July 1984, pp. 111-115.
Systems of the above type are concerned with particle sizes which range from below 0.02 microns (.mu.m) to above 200 .mu.m. Particles with these sizes can be very damaging in semiconductor processing because of the small geometries employed in fabricating semiconductor devices. Typical advanced semiconductor processes today employ geometries which are one-half .mu.m and under. Unwanted contamination particles which have geometries measuring greater than 0.1 .mu.m substantially interfere with 1 .mu.m geometry semiconductor devices. The trend, of course, is to have smaller and smaller semiconductor processing geometries which today in research and development labs approach 0.2 .mu.m and below. In the future, geometries will become smaller and smaller and hence smaller and smaller contamination particles become of interest.
A SMIF system includes a minimum volume, sealed pod used for storing and transporting wafers. Within a wafer fab, a first automated transport system is provided for transferring the SMIF pods from one processing tool bay to another (interbay delivery systems), and a second automated transport system is provided for transferring the pods around within each particular bay (intrabay delivery systems). Each tool bay, typically on the order of about eighty feet long, consists in general of a number of processing tools for performing various wafer fabrication functions, and at least one stocker, where the pods may be stored before or after processing. Additionally, as a pod is generally transferred to several processing tools within a particular bay, the pod may be stored in the stocker between processes. A stocker is typically a large unit having a plurality of shelves on which the pods may be stored, and a transport system for transferring pods into and out of the stocker, and for moving pods around within the stocker.
Some processing tools within a tool bay are typically high throughput tools which are capable of performing their particular wafer process at a relatively higher rate than other processing tools. Additionally, some tools within a bay are metrology tools, which in general monitor or test a single wafer from within a pod of wafers. A pod may store, for example, 25 wafers. If a normal throughput tool can process 50 wafers in an hour, the transport system need only supply 2 pods per hour to that tool. However, for metrology tools which can similarly process 50 wafers in an hour, but only use one wafer per pod, 50 pods must be provided to the metrology tool in an hour to keep the tool from sitting idle.
In order to accommodate high throughput and metrology tools, it is known to include a local tool buffer adjacent the tool port of high throughput and metrology tools, so that pods may be stored locally adjacent such tools and quickly transferred to these tools without having to constantly retrieve a pod from the remotely located stocker. Such local tool buffers are generally configured adjacent the high throughput and metrology tools, and include shelves for storing pods, and a transport system for transferring pods to, from, and within the local tool buffer.
FIGS. 1 and 2 are schematic top and front views, respectively, of a typical tool bay including a stocker 20, and a plurality of processing tools 22 and 40. Additionally, FIGS. 1 and 2 show an interbay delivery system 25 for automated transfer of pods between tool bays, and an intrabay delivery system, explained hereinafter, for automated transfer of pods within a bay. Conventional intrabay delivery systems include a large number of transport interfaces where pods must be handed off, or transferred, from one transport mechanism to another. The transport interfaces within a conventional tool bay will now be explained with reference to FIGS. 1 and 2, and the flow chart shown in FIG. 3. Although shown schematically in the drawings, each interface requires mechanisms for removing or disengaging a pod from a first transport mechanism, and positioning it for engagement with a second transport mechanism. When a pod is first received within a tool bay from the interbay delivery system 25, the pod may either be transferred to the stocker 20 in a step 50, or the pod may be delivered directly to the intrabay transport 30 in a step 52.
Where a pod is first to be stored in the stocker 20, the pod is handed off from the intrabay delivery system 25 to a stocker input/output (I/O) 23 at an interface 27. The stocker I/O 23 in turn hands off the pod to a stocker transport 26 within the stocker at an interface 29 (step 54). Thereafter, a pod may be transferred from the stocker to the tool bay (as explained hereinafter) or back to the interbay delivery system. Where a pod is to be transferred from the stocker back to the interbay delivery system, the pod is transferred from the transport 26 to the stocker I/O 23 in a step 55, and from the stocker I/O, the pod is then transferred to the interbay delivery system 25 in a step 59.
As indicated, the pod may alternatively be transferred directly from the interbay delivery system 25 to the intrabay transport 30 at a mechanical interface 32 in a step 52. From the intrabay transport 30, a pod may be transferred to a plurality of process tools 22, 40, or to the stocker 20 (either before or after transfer to the process tools 22, 40). In order to transfer a pod from the intrabay transport 30 to the stocker, the pod is first transferred to a stocker I/O 21 at an interface 24 in a step 57, and then to the stocker transport 26 at an interface 28 in a step 61. The stocker I/O 21 receiving a pod from the intrabay transport 30 is similar to the stocker I/O 23 receiving a pod from the interbay delivery system, and the stocker transport 26 is capable of receiving pods from both stocker I/O 21 and stocker I/O 23.
Where a pod is to be transferred from the intrabay transport 30 to a processing tool, the pod is transferred to a tool delivery mechanism 34 at an interface 38 (step 60). Alternatively, for the high throughput and metrology tools, such as tools 40, the pod may be transferred in a step 62 first to a local buffer delivery mechanism 42 at an interface 44, and next to an intratool delivery mechanism 46 at an interface 48. The delivery mechanism 46 then positions the pod at a desired location within the local tool buffer. Finally, either the tool delivery mechanism 34 (for normal throughput process tools) delivers the pod to the I/O ports 50 for each tool in a step 63, or the local buffer delivery mechanism 42 (for a high throughput or metrology tool) delivers the pod to the I/O ports 50 in a step 65.
It is further understood that the delivery mechanisms and interfaces as described above are also utilized to return a pod back to the intrabay transport mechanism 30, in steps 64 and 66 for normal throughput process tools, or in steps 68 and 70 for high throughput/metrology tools. The intrabay transport mechanism may then transfer the pods to the stocker, as explained above, or back to the interbay delivery system in a step 67.
As each tool bay may include on the order of approximately 15 to 20 process tools, the overall intrabay delivery system may include over 100 interfaces where a pod must be physically transferred from one transport system to another. Such conventional systems, including large numbers of components and complex hardware, have several disadvantages. First, stockers and/or local tool buffers are expensive, possibly as expensive as the process tools they serve. Also the stockers, local tool buffers, and dedicated transport mechanisms and interfaces take up valuable space within the tool bay. Second, where each tool has a dedicated transport as the sole mechanism for transferring pods to and from that tool, in the event of a transport malfunction, that tool becomes isolated, with no way to feed pods to the tool and no way to remove pods from the tool, other than by manual transfer. Third, each interface where a pod must be handed off from one transport mechanism to another presents a potential danger that a pod will be mishandled during the transfer. The large number of interfaces in conventional systems magnifies the potential for pod mishandling.
In addition to the hardware difficulties presented in conventional delivery and storage systems, the prior art has several disadvantages with respect to the software control of the system. First, each moving and interacting component requires some level of software control. Thus, the many transport mechanisms around the bay, the transport system within the stockers, and the transport systems within the local tool buffers each require software routines for their operation and interaction, resulting in a complex software control system. Second, the software control system must control an interface such that, when a transport mechanism on one side of the interface presents a pod for transfer, the transport mechanism on the other side of the interface must be there to receive it. As explained above, there may be over 100 interfaces within a bay, and an extremely complicated algorithm is necessary to ensure proper timing of a handoff at each interface to avoid either the transferring or receiving mechanism having to wait at an interface. Third, in the event a tool malfunctions, the software control must reroute pods that were scheduled for that tool, remove any pods resident in a local tool buffer for that tool, and find another tool, buffer, or storage location for the pods removed from the tool's local buffer, all without disrupting transfer of pods to and from the other process tools within the bay. Conventional software control programs have extreme difficulty in effectively handling this situation.
As an alternative to the intrabay delivery system shown in FIGS. 1 and 2, it is known to replace intrabay transport 30 with an overhead transport system including a monorail cable hoist. Such overhead transport systems are capable of transporting a pod horizontally along the length of the tool bay, and, when positioned over a desired I/O port 50, cables lower the pod from the transport onto the I/O port 50. Such a system allows the omission of interfaces 38 and 44 shown in FIGS. 1 and 2. However, a problem with such overhead transport systems, as well as a further disadvantage to the conventional system shown in FIGS. 1 and 2, is that there must be a clear, unobstructed path above the I/O ports 50 so that the tool delivery mechanism 34 (in FIGS. 1 and 2) or the cable lift of the overhead transport system may lower a pod onto the I/O port. A further disadvantage to overhead transport systems is that it is important to position the pods in a precise position and orientation on the I/O ports 50, so that the wafers may thereafter be automatedly drawn into the process tools. However, while lowering a pod, cables are susceptible to swaying and have no lateral positioning support other than the force of gravity. Thus, cable systems are prone to error in positioning a pod in the correct position on top of the process tool I/O port.